The two-piece cans are generally DRD (drawn and redrawn can) and DWI (draw and wall ironed can) produced from tin plated steel sheet, aluminium sheet, aluminium alloy sheet or ECCS (electrolytically chromium coated steel), and DTR (drawn thin-redrawn can) is recently put to practical use. DRD has a rather thick wall thickness which becomes thicker in proportion to the can height due to drawing and redrawing. It is then used for a can having a rather low can height from a view point of economy. The materials used for DRD are ECCS, tin plated steel sheet or aluminium alloy sheet. On the other hand, DWI can economically be applied for a can having high can height because the wall thickness can be reduced to one third of that of the original sheet. The material presently used for DWI are tin plated steel sheet or aluminium alloy sheet. But there is great difference between DRD and DWI in respect that the former formed by drawing is made of a metal sheet precoated with an organic coating while the latter formed by ironing is coated after forming. This is attributed to that the processing intensity and the state of stress at the processing are greatly different between DRD and DWI. The application of a metal sheet covered with an organic coating for DWI where the processing intensity (reduction ratio) and the surface pressure to the can wall is extremely great has not yet been put to practical use because of the seizing of the organic coating to the dies or the damage of the organic coatings on the outer and the inner surface of the can.
On the other hand, DTR (drawn and thin-redrawn can) is formed by thinning the can wall which includes bending and bending-back at the corner of drawing dies having a small corner radius and applying a heavy tensile stress. DTR which is formed by the forming resembling drawing has a little thinner can wall thickness than that of the original sheet because the can wall is stretched. In addition, the heavy surface pressure is not applied to the can wall lying between the dies and the punch in the DTR process unlike in the ironing process, then the surface pressure applied is not so heavy and the organic coating is hardly damaged, and then a metal sheet covered with an organic coating can be applied to DTR and ECCS covered with a thermoplastic resin film is industrially used at present. But in the DTR process the can wall is apt to break during the forming because it is mainly formed with a tensile stress, then the wall thickness can only be about 80% of that of the original sheet and is thicker than that of DWI.
As described above, there are advantage and disadvantage in DRD, DWI or DTR, and in the processing of them. Thereupon the objective of the present invention lies in producing a steel sheet of which both sides are laminated with a thermoplastic resin film suitable for the production of a can having a can height about twice the can diameter and a thin wall thickness from 40 to 70% of the original sheet thickness like DWI. The most important objective lies in producing a steel sheet laminated with a thermoplastic resin film which can be formed into a can without the use of emulsion or water soluble lubricant presently used for cooling and lubrication in DWI process. The use of a steel sheet previously laminated with a thermoplastic resin film can omit the coating and baking process in the can producing process, then prevent the difusion of solvent, and then omit the next rinsing, drying and waste water disposal process. There has not been a disclosure relating to a metal sheet laminated with a thermoplastic resin film wherefrom a can having a high can height and a thin wall thickness can be produced without such a water based coolant or lubricant, nor such a can or the process of producing it. However, there are the following disclosures relating to the present invention of which the objective is different from that of this invention.
Laid-Open Japanese Patent Publication No. Sho. 62-275172 shows a metal sheet covered with an organic coating for a two-piece can and the objective of it lies in increasing the retention of coolant (water based cooling and lubricating agent) at the outer surface of a can in DWI process. Namely, it depends on the use of a water based cooling and lubricating agent, then it is different from that of the present invention. Also, WO89/03303 shows a metal sheet for DWI of which one or each side is covered with a polyester resin film, and it also depends on the use of a water based cooling and lubricating agent in the same manner as above-described publication. Also, Laid-Open Japanese Patent Publication No. Hei. 4-91825 shows a metal sheet covered with a thermoplastic resin wherefrom a can having a thin wall is formed by bending and bending-back with a lubricant which volatilizes at a high temperature but without a water based cooling and lubricating agent. It relates to DTR, but as shown in its Examples the reduction rate of the can wall thickness is about 20% and smaller than the goal reduction rate of the present invention.
In the present invention the can height becomes higher in proportion to the increase of the reduction rate of the can wall thickness from 40 up to 70%. This serves the aim of the present invention, but the greater the reduction rate of the can wall thickness is, the more likely the seizure of the outer can wall to the dies and the damage of the resin layer or the break of the wall are. Due to the absence of water based cooling and lubricating agent, the prevention of damage of the resin layer on the outside can wall and the break of the can wall due to it is the most important objective. In addition, it is another important objective to provide enough adhesive strength between the steel sheet and the laminated resin layer because it decreases in proportion to the processing intensity.